A Re-evaluation of the Solubility and Stability Regions of Calcium Arsenites and Calcium Arsenates in Aqueous Solution at 25°C

Abstract

The systems Ca(II)-As(III)-H₂O and Ca(II)-As(V)-H₂O were studied by equilibrating mixtures of calcium oxide and arsenious or arsenic acid solution at 25°C for one month. Where the pH of the final solution was likely to be greater than 8, care was taken to exclude atmospheric carbon dioxide. Chemical analysis of the final solid phases and the associated solution from each experiment for calcium and arsenic, together with X-ray diffraction analysis of the solids and an inspection of graphed relationships, indicated the existence of the compounds Ca(AsO₂)₂ and CaAsO₂OH 1/2H₂O in the system Ca(II)-As(III)-H₂O; and Ca(H₂AsO₄)₂, CaHAsO₄ H₂O, Ca₃(AsO₄)₂ 4H₂0 and Ca₂AsO₄OH 2H₂O in the system Ca(II)-As(V)-H2O. The solubility and stability regions for these compounds were assessed as a function of pH, and solubility constants and conditional free energies of formation for each compound calculated from the solubility data obtained. A brief historical review of literature in which calcium arsenites and calcium arsenates have been reported is included in this paper.